1. Field of the Invention
The present invention relates to an image forming apparatus, and particularly relates to a technique effective for improving an image quality in an electrophotographic system type image forming apparatus.
2. Description of the Prior Art
In recent years, in an electrophotographic system type image forming apparatus, there is a structure which adjusts an optimum value of a transfer voltage applied to a transfer roller on the basis of a temperature and a humidity detected by a temperature and humidity detecting sensor in order to obtain a stable transfer quality even under an environment where a temperature, a humidity and the like momentarily change.
There is a difference between a transfer voltage optimum for properly transferring multi-layered toners such as a color image and the like and a transfer voltage optimum for properly transferring a single-layered toner such as a monochrome image and the like, and the respective optimum voltages are changed in accordance with a change of an environment (a temperature and a humidity). In particular, in an image in which a multi-layered toner image and a single-layered toner image simultaneously exist, since the optimum transfer voltage is different in each of the layers, it is necessary to change the transfer voltage in a further fine manner in accordance with the respective environments. Further, the optimum transfer voltage also changes in accordance with a value of volume resistance and a value of surface resistance of a recording medium (a paper, a plastic film and the like).
Then, in a conventional technique, it is structured such that the optimum transfer voltage is determined by performing a printing test under various kinds of environments such as every recording mediums, every printing modes, for example, one-sided printing, both-sided printing and the like, every printing speeds and the like, an environment near a receiving place of the recording medium within a color electrophotographic image forming apparatus is detected by the temperature and humidity detecting sensor, and the transfer voltage is controlled in accordance with a kind and a temperature and humidity of the recording medium.
Accordingly, in the conventional color electrophotographic image forming apparatus, it is necessary to perform a printing test under the various kinds of environments such as every recording mediums, every printing modes, every printing speeds and the like so as to determine the optimum transfer voltage. Then, the more the conditions such as the recording mediums, the printing modes, the printing speeds, the temperature and humidity and the like become increased, the more a recording amount of a memory becomes increased.
In this kind of conventional color electrophotographic image forming apparatus, in order to reduce a memory capacity as much as possible, or in order to reduce a number of the printing tests as much as possible, it is preferable to prepare desultory transfer voltage data at every predetermined temperatures or every predetermined humidity, that is, the transfer voltage data with respect to discontinuous temperature and humidity in place of preparing the transfer voltage data corresponding to all the temperatures and humidity.
However, in the case that only these discontinuous data are prepared as mentioned above, there is a problem that a large difference is generated between a proper transfer voltage and an actual transfer voltage near a boundary of the defined temperature and humidity, thereby deteriorating an image quality. In particular, in the case that the temperature and the humidity gradually change in response to a lapse of time, there is a case that an image quality is significantly deteriorated when the temperature or the humidity becomes over a certain temperature or a certain humidity.